Accompanying higher density recording in optical disks such as DVDs, not only DVD drives for reproduction but also DVD drives for recording information in DVD-RAMs, DVD-RW or the like have become commercialized. Also, their recording speed keeps increasing.
In response to such an increasing recording speed of the DVD drives for recording, there has been a demand for higher power semiconductor lasers used as their light source. As an effective means of achieving the higher power of semiconductor lasers, various suggestions have been made; one of which is to process a cladding layer above an active layer, thus forming a ridge-shaped stripe having high perpendicularity and high symmetry. Incidentally, having high perpendicularity and high symmetry means that ridge lateral wall surfaces (lateral surfaces) are substantially perpendicular to a semiconductor substrate surface in a cross-section perpendicular to a longitudinal direction (stripe direction) of the ridge and that the ridge has an excellent right-left symmetrical cross-section, respectively. In the present invention, the cross-section perpendicular to the ridge stripe direction means a cross-section that crosses the longitudinal direction of the ridge at a right angle.
By improving the perpendicularity and symmetry of the ridge shape in the cross-section perpendicular to the longitudinal direction (stripe direction) of the ridge and controlling an electric current distribution profile and an optical distribution profile to be equivalent, it is possible to improve a kink level, which is necessary for achieving a higher power. Further, by equalizing a top dimension and a bottom dimension of the ridge substantially, a thermal resistance at the time of current injection can be reduced, thereby achieving a low operating current.
However, in the case of a visible light semiconductor laser with an emission wavelength band of 650 nm, for example, in order to suppress the formation of a natural superlattice (an ordered structure) of a GaInP layer, a semiconductor substrate that is off-angled by about 10° in a [011] direction from a (100) plane is used in general. When the ridge-shaped stripe is formed using a wet etching technique, the ridge shape in the cross-section perpendicular to the longitudinal direction (stripe direction) of the ridge reflects the off-angle of the substrate and becomes right-left asymmetrical. Also, since a side etching amount of the cladding layer with respect to an etching mask is large in the wet etching, the ridge shape in the cross-section perpendicular to the longitudinal direction (stripe direction) of the ridge becomes a trapezoidal shape with its wall surfaces having low perpendicularity. From the above, it has been very difficult to solve the asymmetry of the ridge shape and improve the perpendicularity thereof in the cross-section perpendicular to the longitudinal direction (stripe direction) of the ridge.
In recent years, a technology has been suggested in which a ridge-shaped stripe is formed using both dry etching and wet etching, thus improving the perpendicularity and symmetry of the ridge shape in the cross-section perpendicular to the longitudinal direction (stripe direction) of the ridge (see Patent document 1 listed below, for example). Since the dry etching is capable of an anisotropic etching, it achieves a ridge shape having improved perpendicularity and symmetry in the cross-section perpendicular to the longitudinal direction (stripe direction) of the ridge compared with the case of forming the ridge-shaped stripe by wet etching alone. Also, by the wet etching after the dry etching, a damaged layer caused by plasma at the time of the dry etching is removed.
Moreover, in order to improve the perpendicularity and symmetry of the ridge shape in the cross-section perpendicular to the longitudinal direction (stripe direction) of the ridge, a technology of forming a ridge-shaped stripe by dry etching alone has been suggested (see Patent document 2 listed below, for example). This technology makes it possible to achieve a ridge shape having improved perpendicularity and symmetry in the cross-section perpendicular to the longitudinal direction (stripe direction) of the ridge compared with the case of forming the ridge-shaped stripe using both dry etching and wet etching.
Herein, a semiconductor laser device in the conventional technology illustrated in Examples 1 and 3 of Patent document 1 and a method for manufacturing the same will be described, with reference to FIGS. 3 and 4. FIG. 3 is a sectional view showing a structure of the semiconductor laser device described in Examples 1 and 3 of Patent document 1, and FIG. 4 is a sectional view showing its production processes, both viewed from a direction perpendicular to the longitudinal direction of the ridge-shaped stripe.
As shown in FIGS. 3 and 4(a), an n-type AlGaAs cladding layer 303, an active layer 304 with a quantum well structure, a p-type AlGaAs cladding layer 305, a p-type AlGaAs etching stop layer 306, a p-type AlGaAs cladding layer 307 and a p-type GaAs cap layer 309 are grown epitaxially in this order on an n-type GaAs substrate 301 by metal-organic chemical vapor deposition (in the following, referred to as MOCVD) (note that, in FIG. 4, layers corresponding to the n-type GaAs substrate 301, the n-type AlGaAs cladding layer 303 and the active layer 304 with the quantum well structure in FIG. 3 are omitted). Thereafter, photoresist is applied onto a surface of the p-type GaAs cap layer 309, and a ridge-shaped stripe pattern 313 of the photoresist is formed by a photolithography technique.
Here, in the case of producing an AlGaInP red semiconductor laser device, a p-type intermediate layer (for example, a p-type GaInP intermediate layer) is deposited between the p-type cladding layer 307 and the p-type GaAs cap layer 309 (not shown).
Although the ridge-shaped stripe pattern 313 is formed using the photoresist, it also may be formed using a dielectric material such as SiO2.
Next, as shown in FIG. 4(b), the p-type GaAs cap layer 309 and the p-type cladding layer 307 are etched by a dry etching technique to a position 50 nm to 350 nm above the p-type etching stop layer 306 formed under the p-type cladding layer 307.
Then, as shown in FIG. 4(c), wet etching is carried out until the p-type etching stop layer 306 is reached, thus forming a ridge-shaped stripe formed of the p-type AlGaAs cladding layer 307 and the p-type GaAs cap layer 309.
Subsequently, as shown in FIG. 4(d), after removing the photoresist 313, an n-type current blocking layer 310 is deposited by MOCVD, and then the current blocking layer in a current injection region, namely, on a surface of the p-type GaAs cap layer 309 is removed by wet etching. Thereafter, a p-type GaAs contact layer 311 is formed by MOCVD again, thus completing a semiconductor laser wafer (see FIG. 3 for the completed product).
By the manufacturing method described above, the ridge shape with relatively improved perpendicularity and symmetry in the cross-section perpendicular to the longitudinal direction (stripe direction) of the ridge can be achieved for the AlGaAs infrared semiconductor laser device and the AlGaInP red semiconductor laser device. Also, with the wet etching, it is possible to control an etching depth and remove a layer damaged by plasma at the time of dry etching.
Next, a semiconductor laser device in the conventional technology illustrated in Example 2 of Patent document 1 described above and a method for manufacturing the same will be described, with reference to FIGS. 5 and 6. FIG. 5 is a sectional view showing a structure of the semiconductor laser device described in Example 2 of Patent document 1, and FIG. 6 is a sectional view showing its production processes, both viewed from a direction perpendicular to the longitudinal direction of the ridge-shaped stripe.
As shown in FIGS. 5 and 6(a), an n-type AlGaAs cladding layer 503, an active layer 504 with a quantum well structure, a p-type AlGaAs cladding layer 505, a p-type etching stop layer 506, a p-type AlGaAs cladding layer 507 and a p-type GaAs cap layer 509 are grown epitaxially in this order on an n-type GaAs substrate 501 by MOVPE (note that, in FIG. 6, layers corresponding to the n-type GaAs substrate 501, the n-type AlGaAs cladding layer 503 and the active layer 504 with the quantum well structure in FIG. 5 are omitted). Thereafter, a dielectric material such as Al2O3 is deposited onto a surface of the p-type GaAs cap layer 509, and a ridge-shaped stripe pattern 513 of the above-mentioned dielectric material such as Al2O3 is formed as a mask by a photolithography technique.
Here, the p-type etching stop layer 506 is an In-containing layer having a band gap that does not absorb a laser beam or an In-containing layer having a thickness designed for obtaining a quantum effect. For example, it is an AlGaInP layer or a GaInP layer.
Next, as shown in FIG. 6(b), the p-type AlGaAs cladding layer 507 and the p-type GaAs cap layer 509 are dry-etched until the p-type etching stop layer 506 is reached.
Since an inductively coupled plasma method (in the following, referred to as an ICP method) is used for the dry etching and the In-containing layer is used as the p-type etching stop layer 506, the etching rate in this layer considerably is lower than that in the p-type AlGaAs cladding layer 507 and the p-type GaAs cap layer 509. Thus, in the dry etching, the etching can be stopped in the etching stop layer 506.
Subsequently, as shown in FIG. 6(c), after removing the mask of the ridge-shaped stripe pattern 513 formed of the dielectric material such as Al2O3 described above with a chemical solution containing hydrofluoric acid as a principal component, a current blocking layer 510 is formed by MOCVD. Then, an unwanted portion of the current blocking layer 510 grown on the ridge-shaped stripe is removed by a photolithography technique using a photoresist, followed by forming a p-type GaAs contact layer 511 by metal-organic vapor-phase epitaxy (in the following, referred to as MOVPE), thus completing a semiconductor laser wafer (see FIG. 5 for the completed product).
In dry etching, since sputtering, which is a physical phenomenon, is a prime factor, it is difficult to secure a sufficiently large selectivity such as that achieving a sufficient difference in an etching speed between materials. However, using the etching stop layer containing In, the above-described manufacturing method secures a selectivity in the dry etching. In this manner, the ridge with high perpendicularity and high symmetry is formed by the dry etching alone.
Next, a semiconductor laser device in the conventional technology illustrated in Patent document 2 and a method for manufacturing the same will be described, with reference to FIGS. 7 and 8. FIG. 7 is a sectional view showing a structure of the semiconductor laser device described in Patent document 2, and FIG. 8 is a sectional view showing its production processes, both viewed from a direction perpendicular to the longitudinal direction of the ridge-shaped stripe.
As shown in FIGS. 7 and 8(a), an n-type (Al0.7Ga0.3)0.5In0.5P cladding layer 703, a GaInP/AlGaInP multiple quantum well active layer 704, a p-type (Al0.7Ga0.3)0.5In0.5P cladding layer 707, a p-type GaInP hetero buffer layer 708 and a p-type GaAs cap layer 709 are grown epitaxially in this order on an n-type GaAs substrate 702 by MOCVD. Thereafter, an SiO2 film is formed on an entire surface of the substrate, and an SiO2 stripe 713 is formed by a photolithography technique.
Next, as shown in FIG. 8(b), using the SiO2 stripe 713 as a mask, the p-type GaAs cap layer 709, the p-type GaInP hetero buffer layer 708 and a part of the p-type (Al0.7Ga0.3)0.5In0.5P cladding layer 707 are etched by a dry etching technique, thus forming a ridge-shaped stripe.
Then, as shown in FIG. 8(c), using the SiO2 stripe 713 as a mask, an n-type AlInP current blocking layer 705 and an n-type GaAs current blocking layer 706 are grown epitaxially in this order by MOCVD.
Subsequently, as shown in FIG. 8(d), the SiO2 stripe 713 is removed, and a p-type GaAs contact layer 710 is grown on the entire surface of the substrate by MOCVD. Finally, a p-side electrode 711 and an n-side electrode 701 are formed, thus producing the semiconductor laser device.
By the manufacturing method described above, a ridge-shaped stripe can be formed by the dry etching alone, thus achieving a ridge shape with high symmetry and perpendicularity in the cross-section perpendicular to the longitudinal direction (stripe direction) of the ridge.    Patent document 1: JP 2003-69154A    Patent document 2: JP 2000-294877 A